1. Field of the Invention
The present invention relates to a polishing apparatus and a polishing method for a substrate, such as a wafer, and more particularly to a polishing apparatus and a polishing method capable of detecting a polishing end point of a substrate.
2. Description of the Related Art
Various types of polishing end point detection methods have been used in apparatus for polishing a substrate, such as a wafer. For example, in order to detect a point at which an upper film is removed by polishing of it and as a result a lower film is exposed, a method of detecting a change in a torque current of a polishing table is used (for example, see Japanese laid-open patent publication No. 2001-198813 and Japanese laid-open patent publication No. 6-315850).
As interconnects have been becoming finer, a more accurate detection of the polishing end point is required. However, the above-mentioned method of detecting the polishing end point based on the torque current may result in excessive polishing of a wafer if there is a variety in thickness of the upper film over the wafer surface. Specifically, if the wafer is polished until the lower film is exposed over the wafer surface in its entirety, the lower film may be polished excessively with respect to a target film thickness.
In order to prevent such excessive polishing, there is proposed a method in which the wafer is polished for a predetermined time from a point at which removal of initial irregularities formed on a surface of the upper film is detected from the change in the torque current. This method includes the steps of terminating polishing of the wafer when the remaining lower film is thicker than its target thickness, measuring the film thickness in an exterior film thickness measuring device, calculating a polishing time necessary to eliminate a difference between the target film thickness and the measured film thickness, and additionally polishing the wafer for the calculated polishing time to achieve the target film thickness. However, this method includes the additional polishing of the wafer, which increases a whole polishing time and lowers a throughput.
Other than the method of detecting the polishing end point based on the torque current, there is a method of detecting the polishing end point using an optical sensor (for example, see Japanese laid-open patent publication No. 2004-154928). This type of method includes the steps of directing a light at the surface of the wafer, and analyzing a reflected light from the wafer to determine the polishing end point of the wafer. According to this method, it is possible to terminate polishing of the wafer before the lower film is exposed, because the polishing end point is detected from a polished state of the upper film. However, interconnect patterns formed in the wafer or slurry used in the polishing of the wafer may adversely affect the accuracy of the polishing end point detection, and as a result the required accuracy may not be achieved.